The present invention relates to a motor control system, and more particularly to a circuit for controlling the rotational speed of a DC motor.
The rotational speed of a DC motor is proportional to the counterelectromotive voltage. Accordingly, the principle for a speed control of a motor is to change a motor driving current in accordance with variation of a motor load and to counteract, by controlling a voltage between terminals of the motor, the variation of a voltage drop across an internal resistance of the motor caused by the change of the motor driving current. Thus, the counterelectromotive voltage of the motor becomes constant, and the rotational speed thereof is stabilized against load variation.
A speed control circuit of a DC motor is based upon the above-mentioned operating principle. However, the control circuit according to prior art has a drawback that the variation of a bias current due to the changes of a power source voltage and/or an operating temperature causes the counterelectromotive voltage of the motor to vary with the result of change in a rotational speed.